The present invention relates to a method and apparatus for connecting a well on the ocean floor with a wellhead "Christmas" tree, (i.e., the flow control valves) on a fixed or relatively fixed platform, such as a floating tension leg platform, or the like. More particularly, the present invention relates to a method and apparatus used in connecting a riser tensioner system between the riser and the relatively fixed platform so that pairing of tensioners is unnecessary to avoid torsional loading of the riser in the event of failure of one of the tension cylinders.
One of the benefits of a tension leg platform over other floating systems is the very small vertical oscillation that occurs. This enables the welhead trees to be mounted within a few feet of a platform deck without the need for some complex form of motion compensation system. However, the use of a rigid riser system requires that a riser tensioner system be employed to compensate for the small amount of platform movement that does take place so that buckling or bending of the riser under its own weight will not result in a failure (cracking, breaking, etc.) of the riser. Heretofore, tensioner cylinders have typically been paired so as to disable the opposite cylinder when one tensioner cylinder failed and thereby avoid unbalanced loading that can torque the riser and produce the failure of the riser the tensioning system is trying to avoid.
The present invention provides the desired motion compensation and tensioning of the riser by angulating each of the plurality of tensioner cylinders to operate through a common point lying along the centerline of the riser below the point of connection to the platform. The piston rod of each cylinder is connected to a tensioner ring that is in turn clamped to the riser. The tensioner ring has a plurality of arms each of which receives a piston rod, extending from its body at an angle that is generally equivalent to the average angle (i.e., the midpoint extension of the piston rod) that the respective tensioner forms with the riser. In this manner, the tensioner ring arm provides a reaction surface that is generally perpendicular to the action line of force exerted on the riser which lies along the piston cylinder and piston rod. By this configuration, the cylinders need not be hydraulically paired in opposing couples. Failure of one cylinder will not result in any torsional forces that are perpendicular to the longitudinal axis of the riser being applied to it, as occurred with many prior art tensioners.
Various other features, advantages and characteristics of the present invention will become apparent after a reading of the following specification.